A Darlington amplifier typically includes an input transistor and an output transistor connected in such a way that the current amplified by the input transistor is amplified further by the output transistor. Such a design has a much higher common/emitter current gain than each transistor taken separately. In the case of integrated devices, a Darlington amplifier can take less space than two individual transistors because it employs a shared collector. Integrated Darlington amplifiers may come packaged singly as three terminal style transistor packages, or as an array of devices on an integrated circuit chip.
Without adequate bias control mechanism, the bias current through a Darlington amplifier will be sensitive to process, temperature, and supply voltage variations. This can be problematic because the bias current determines the gain, output power and distortion characteristics of the Darlington amplifier.
One conventional method to set the bias current Darlington amplifier is to use an off-chip bias resistor as part of a negative feedback network. The bias resistor senses bias current and induces a voltage drop across it that is proportional to the current. The voltage drop reduces the voltage across the Darlington amplifier thus regulating the bias current. The larger the voltage drop across the bias resistor, the higher the negative feedback loop gain and more stable the bias current is and less sensitive the Darlington amplifier will be to supply voltage, process, and temperature variations. However, a larger voltage drop across the bias resistor means there may be less voltage applied to the collectors of the input and output transistors. This reduces the voltage available for the output signal voltage swing. The reduced voltage swing headroom for the output signals may affect saturation output power, compression, third order intercept point (IP3) and other distortion characteristics. Alternatively, voltage swing headroom may be restored by using a higher supply voltage, but this may not be practical and/or it may result in undesirably high DC power dissipation.
Another conventional technique used to bias a Darlington amplifier is disclosed in U.S. Pat. No. 6,927,634, incorporated by reference herein. As disclosed therein, an additional active device, e.g., a transistor, QBIAS, in the form of a current mirror is used to sense the current in the output transistor of the Darlington amplifier. As the current in the output device increases, the voltage at its base increases which is transferred to the base of the active transistor QBIAS. The increase in the base voltage of QBIAS produces a proportional increase in current. This increase in current increases the voltage drop across the feedback resistor which reduces the voltage at the base of the input transistor. This reduces the current in the input and output transistors of the Darlington amplifier to effectively control the bias current.
Because the active device as disclosed in the '634 patent is placed in the signal path of the amplifier, it needs to be isolated with inductors and capacitors so that it does not interact with the input and output transistors. The effectiveness of such isolation components is dependent on frequency. The result may be degradation of the noise figure of the Darlington amplifier